1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus, an image processing apparatus, and an image processing method used to generate a polar map related to functional indices about organs, such as a heart.
2. Description of the Related Art
Speedups in helical scan and developments in volume scan using a 2-D detector have greatly changed the concept of an X-ray computed tomographic apparatus from a mere cross-section imaging apparatus to a volume imaging apparatus. In other words, an X-ray computed tomographic apparatus adopting these scan methods has enabled fast scan of a subject in volume at a short temporal resolution. Practical applications of an X-ray computed tomographic apparatus equipped with these functions have found a way into many diagnostic fields where the X-ray computed tomographic apparatus has not been used, and an effort to find a novel diagnostic method is being made.
One of the possible directions is adaptation to evaluation diagnosis on palmic motions of the heart. Together with the segment scan method, the volume scan substantially achieves successive scanning of the entire heart in short cycles. This makes it possible to generate, for example, a stereoscopic image of the heart limited in the end of diastole (ED) or the end of systole (ES). Further, it is possible to find cardiac functional indices, such as wall thickening from ED to ES, across the entire heart. The heart extends longitudinally from the apex to the base, and a polar map is used to enable observation of a distribution of the cardiac functional indices across the longitudinal heart at a glance. As is known, a polar map is an expression method of a plane by developing data on polar coordinates expressed as (r, θ). In the case of the heart, θ is given as an angle about the myocardial axis, and r is given as a distance to each slice in a direction from the apex to the base of the heart.
A polar map makes it easy to observe a portion causing deterioration in the cardiac function, for example, by a difference in display colors. When used in a diagnosis actually, a relation between the deteriorated portion and a dominant blood vessel is needed. Hence, as shown in FIG. 20, a polar map, when displayed, is often superimposed with anatomically divided, established typical dominant regional segments. A segment denoted by “LAD” represents a dominant region of the left anterior descending branch, a segment denoted by “LCX” represents a dominant region of the left circumflex branch, and a segment denoted by “RCA” represents a dominant region of the right coronary artery.
By displaying a polar map superimposed with the typical dominant regional segments as has been described, it is possible to understand in which of the left anterior descending branch, the left circumflex branch, and the right coronary artery a lesion is caused in a coronary artery, that is, in which dominant region of an artery a lesion is caused; however, place and location of a lesion in the coronary artery cannot be specified.